Was Einstein Wrong? Tomorrow Begins A Seven-Year Quest To Mercury To Refine Spacetime

BepiColumbo will shed light on when the innermost planet Mercury formed, and its strange chemical composition.

ESA/ATG medialab

We know everything there is to know about the planets in the solar system, don't we? Though all of the planets have been visited or photographed at least once by a spacecraft, Mercury – one of the closest to Earth – is the least explored and least understood.

That will all change tomorrow if, as planned, the BepiColumbo mission is launched on top of an Ariane 5 rocket from Europe’s Spaceport in Kourou in French Guiana, South America.

It could see Einstein's general theory of relativity examined in excruciating detail.

Have we been to Mercury before?

Yes. NASA's Mariner 10 photographed it in 1974-75, and MESSENGER mapped it from 2008-2015. Now that ESA and JAXA have the map, they want to know more about Mercury. So much so that BepiColombo is actually three separate spacecraft.

Why is it called BepiColombo?

The spacecraft is named after the late Professor Giuseppe (Bepi) Colombo, an Italian mathematician and engineer who discovered a resonance that makes Mercury rotate on its axis three times every two years. He also helped NASA use gravity-assist swing-bys of Venus and Mercury for its Mariner 10 probe.

London's Science Museum currently has a full-size 6m high engineering model of BepiColombo.

Jamie Carter

What will BepiColombo study?

It's a pretty ambitious science program. First proposed in 1993, BepiColombo's two orbiters will together study Mercury's origin and evolution, its interior structure, geology, composition and craters, its atmosphere and magnetosphere (and how it interacts with the solar wind), the origin of Mercury's magnetic field, and examine deposits at its poles. BepiColombo will also map of Mercury at different wavelengths.

Why are there three spacecraft?

There's one spacecraft carrying two orbiters; the ESA Mercury Planetary Orbiter (MPO) and the JAXA Mercury Magnetospheric Orbiter (MMO). Although both will orbit separately, they'll work in-sync to give astronomers two data points on some projects. The MPO will mostly study Mercury's surface and composition, while the MMO will focus on Mercury's magnetic environment and the chemical make-up its thin exosphere (to call it an atmosphere would be over-playing it). However, they'll be able to simultaneously study how the solar wind affects both the planet's exosphere and its surface, something that requires two orbiters.

How will it test Einstein's theory of general relativity?

Albert Einstein's theory of general relativity describes gravity as a symptom of spacetime being warped, twisted and curved by the presence of massive objects, such as stars and planets. So the Sun warping spacetime explains the curved orbits of the planets, which are actually falling around the Sun. In a May 2018 paper called New General Relativistic Contribution to Mercury’s Perihelion Advance, author Will Clifford at the Department of Physics, University of Florida, argues that Mercury’s orbital path shifts a degree every two billion years because of an indirect effect of general relativity. That's a pretty subtle figure, but this as-yet-untested theory – that the pull between the Sun and Jupiter (and between the Sun and each of the other planets) also very slightly affects the orbits of all the other planets – could be detected by BepiColombo. Since Mercury it's the closest planet to the Sun, the effect is expected to be most easily detectable at Mercury.

BepiColombo's Mercury Transfer Module, which will carry the two orbiters to Mercury via Earth and Venus in a seven-year journey.

ESA/ATG medialab & NASA/JPL

Why is Mercury so interesting?

We don't know how a planet formed and evolved so close to the Sun, and nor do we understand Mercury's weird chemistry. Two studies presented last month at the European Planetary Science Congress (EPSC) 2018 in Berlin highlighted why BepiColombo is so important. Firstly, it may have formed very early in the solar system's history from condensed vapor from planetesimals. "We think that very early in the solar system, planetesimals in the innermost region of the solar system could have formed from re-processed material that was vaporized due to the extreme temperature there, and subsequently recondensed," said Thomas Ronnet at the University of Aix Marseille. Meanwhile, his colleague Bastien Brugger ran computer simulations of Mercury’s interior that suggest that it has a dense mantle that may contain lots of iron. "With the launch of BepiColombo, we will have a whole new suite of instruments to continue the investigation of Mercury's unique properties, and try to better understand the structure and origin of the planet."

The interior of Mercury, which BepiColumbo will study.

Brugger/ University of Aix Marseille

How will BepiColombo get to Mercury?

Just over seven years, but the route it's taking is complex. Although it will use a combination of solar power and electric propulsion, fittingly (given its namesakes' work in gravity assists), BepiColombo will reach Mercury at the correct velocity thanks to flybys first of Earth, then twice from Venus, and six times from Mercury itself. However, when it finally gets there BepiColombo will still require thrust from chemical propulsion engines to slow down enough to insert the MPO into its orbit.

A false-color image of Mercury.

NASA/JHU-APL/Carnegie Institution of Washington

Can I see Mercury in the night sky?

Since Mercury is the closest planet to the Sun in our solar system, as seen from Earth this inner planet is only visible just before dawn or just after dusk. This month it's behind the Sun, though it will re-emerge above the western horizon just after sunset on November 6, 2018.

BepiColombo is due to orbit Mercury for around one Earth year (which is about four Mercury years) starting December 5, 2025. However, it's first Mercury flyby will be October 2, 2021.

After all, it takes time to travel through space, but it takes even longer to slow down.